Materials that are gaseous at standard temperature and pressure are often contained for storage and/or transport within pressure vessels. Such pressure vessels allow a large mass quantity of the gas to be stored/transported in a relatively small space. Upon compression of the gas, some gases will typically transition to a liquid. At least a portion of the container will typically still contain gaseous vapors even when a majority of the material is in a liquid state within the container. Together the gaseous vapors and/or fluids are referred to herein as “high pressure fluid.”
Pressure vessels must be carefully designed and carefully manufactured to avoid rupture and the associated potential for explosion or other hazards associated with reaction forces acting on portions of the tank and/or the potential for fire and wide distribution of potentially hazardous gas into a surrounding environment. Such design for the pressure vessels includes selection of appropriate materials for handling the fluid under pressure, and also selecting proper manufacturing techniques for the vessels. Furthermore, any valves, sensors or other equipment coupled to the vessel must similarly be carefully designed and manufactured so that the vessel can safely and effectively contain the fluid at high pressure and monitor conditions within the vessel.
Typically the design and manufacture of pressure vessels involves a large safety factor so that the pressure vessel can actually contain the fluid at significantly higher pressures than the pressure of the gas actually stored, or transported within the vessel. For instance, a pressure vessel designed to contain fluid at 3,000 psi might in fact be able to contain the fluid at up to 6,000 psi before a rupture would occur, providing a safety factor of two. Often actual safety factors of three or more are specified by regulatory authorities.
One condition which can cause a pressure vessel to exceed its design pressure is when excessive heat, such as that associated with a fire, is experienced by the pressure vessel. When a gas (in its gaseous phase) undergoes heating, its pressure increases, with pressure being proportional with temperature when held at constant volume, such as within a pressure vessel, according to the ideal gas law. If sufficient heating occurs, the design pressure and the margin of safety can both be exceeded, leading to potential explosive rupture of the tank. Vessels often include a “burst diaphragm” which is designed to fail at a pressure slightly less than the maximum pressure the vessel can contain. Thus, if the pressure approaches the maximum pressure the burst diaphragm will burst in a more controlled fashion and relieve pressure before the vessel explodes.
Burst diaphragms are not entirely reliable when the vessel is undergoing heating, such as in a fire. Typically, when a high heat source such as a fire is acting on the pressure vessel, other damage may have occurred to the pressure vessel weakening the pressure vessel somewhat, and the temperature itself has a direct affect on the strength of the pressure vessel, potentially weakening the materials forming the pressure vessel. If the vessel's strength is reduced to below the strength of the burst diaphragm, uncontrolled explosive rupture may occur.
Experience has shown that it is best to allow fluids contained within a pressure vessel to bleed off in a somewhat controlled fashion rather than to allow pressure to build up within a pressure vessel to the point where an explosive rupture might occur. Furthermore, experience has shown that when high temperature is acting on a pressure vessel, a pressure based relief valve or burst diaphragm alone is often not sufficient.
Often pressure vessels do not include a readily available power source and operate in environments where it is undesirable to utilize powered temperature sensors, such as a thermal couple acting on a solenoid driven pressure relief valve. Also, in a fire such power sources typically fail. Thus, it is desirable to provide simple solid state relief devices of a purely mechanical nature, and which reliably remain closed except when a high temperature is experienced above a threshold temperature, to open the relief device when such a high temperature is reached and to relieve pressure from the pressure vessel. One known way to provide such temperature activated devices is to utilize a eutectic material that has a melting point at the threshold temperature desired for the tank. This eutectic material is placed within a vent circuit. When a temperature above the melting point for this eutectic material is reached, the eutectic material melts and fluid is allowed to escape out of the vent. Such systems are common with fire sprinklers plumbed into buildings.
While such pressure relief devices are generally effective, eutectic materials do not handle exposure directly or indirectly to high pressure particularly well. In particular, commonly available eutectic materials are subject to a relatively high degree of creep when loaded under high pressure. This creep over time can cause a pressure vessel to be vented even though no high temperature has been experienced. While such pressure relief devices can merely be replaced on a regular basis, this creates a burden for the pressure vessel operator. Accordingly, a need exists for a thermally activated pressure relief device which is not subject to the disadvantage of creep acting on a eutectic material anywhere within the pressure relief device, but which still can operate in a variety of different environments without requiring a power supply, sensors, scheduled maintenance or complex electrical or mechanical components.